Antivirus software is somewhat of a necessity if you're a Windows user, but the software you choose really does matter, and one app might not be enough. Security expert Brandon Gregg believes that your best bet is a combination of Microsoft Security Essentials (our pick) and a free or open-sourced product. Here's why.

Windows has more antivirus programs than we can count, and none of them are quite perfect. Right…
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While you should never run two antivirus programs at the same time, having two on your system can be beneficial. Most antivirus software (e.g. McAffee, Norton) try to track down viruses on the web and elsewhere to identify them, then send you an update with virus definitions so your system can detect potential threats. This might seem good, but Brandon identifies two (of many) major issues with this approach to antivirus software:

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Issue 1: An advance persistent threat (APT) (aka China or a talented black hat hacker or the NSA) makes a 100% custom, never seen before virus (Stuxnut anyone?) and sends it to one system (Iran?) instead of blanketing it on the internet for McAfee to stumble upon. Do you think on one targeted machine McAfee will find it and then send out a signature file for the rest of us? Probably not.

Issue 2: I can confirm McAfee (and others big boy virus companies) white list (ignore) viruses for law enforcement and other intelligence agencies (including private companies). So you could have a keylogger, Remote Access Tool or other malware on your machine that McAfee is allowing!

Instead of using major software, Brandon suggests that you use Microsoft Security Essentials as your main, always-on antivirus program, along with running a free or open-sourced product once a week. Free products, like MalwareBytes, don't necessarily suffer from bureaucracy or a corporate agenda. Neither do open-sourced products, and they also benefit from crowd sourcing. Basically, Microsoft Security Essentials is great, but you might want to add a second program to your arsenal to make sure Microsoft is doing its job.

Again, The important thing is to make sure you do not run both applications simultaneously. Antivirus programs do not usually play well together, and running two at the same time will often lead to one singling out the other as a virus, or in some cases, even cause file corruption. Plus, running two apps can really eat up system resources. Instead, run Microsoft Security Essentials regularly and run your secondary option manually—with Microsoft Security Essentials turned off—once a week. That way you can get a that extra security without mucking up your system.